The present invention relates to a highly efficient and compact DC conversion apparatus such as a DC/DC converter.
FIG. 1 is a circuit configuration diagram of a conventional DC conversion apparatus. In the DC conversion apparatus shown in FIG. 1, a synchronous rectifier that includes a low on-resistance power MOSFET (Field-Effect Transistor) and the like is used on a secondary side (output side) of a transformer in order to reduce power losses.
In FIG. 1, a MOSFET (Field-Effect Transistor) switch Q1 is connected to a DC power source vdc1 via a primary winding 5a (the number of turns of n1) of a transformer T1, and a series circuit of a resistor R1 and a capacitor C1 is connected to both ends of the switch Q1. A series circuit of a diode D2 and a capacitor C2 is connected to both ends of the primary winding 5a of the transformer T1, and a resistor R2 is connected to both ends of the capacitor C2. The switch Q1 is turned ON/OFF by PWM control of a control circuit 10.
The primary winding 5a of the transformer T1 and a secondary winding 5b (the number of turns is n2) of the transformer T1 each have a winding structure such that a common mode voltage is generated mutually, and a MOSFET switch Q3 and a MOSFET switch Q4 are connected in series to both ends of the secondary winding 5b of the transformer T1. One end of the secondary winding 5b (at filled circle (●) side) of the transformer T1 is connected to a gate of the switch Q4, and the other end thereof is connected to a gate of the switch Q3. The switch Q3 is connected to a diode D3 in parallel, and the switch Q4 is connected to a diode D4 in parallel. These devices form a synchronous rectifying circuit. This synchronous rectifying circuit rectifies a voltage (ON/OFF-controlled pulse voltage) generated at the secondary winding 5b of the transformer T1 in synchronization with ON/OFF operations of the switch Q1, and outputs a DC voltage.
A smoothing reactor Lo and a smoothing capacitor Co are connected in series to both ends of the switch Q3 to form a smoothing circuit. This smoothing circuit smoothes a rectified output of the synchronous rectifying circuit, and outputs a DC output to a load RL.
The control circuit 10 ON/OFF controls the switch Q1 so that the width of an ON-pulse applied to the switch Q1 is narrowed when the output voltage of the load RL reaches or exceeds a reference voltage. That is, when the output voltage of the load RL reaches or exceeds the reference voltage, the width of the ON-pulse applied to the switch Q1 is narrowed so as to control the output voltage to a constant voltage.
Operations of the DC conversion apparatus thus configured are explained with reference to a timing chart at light load time shown in FIG. 2. In FIG. 2, a drain-source voltage of the switch Q1 is depicted by Q1v, a drain current of the switch Q1 is depicted by Q1i, a drain current of the switch Q3 is depicted by Q3i, a drain current of the switch Q4 is depicted by Q4i, a drain-source voltage of the switch Q3 is depicted by Q3v, and a gate voltage signal of the switch Q1 is depicted by Q1g. 
Operations performed under high load are explained first. When the switch Q1 is turned ON by the gate voltage signal Q1g, the current Q1i flows through a path passing along Vdc1, 5a, Q1, and Vdc1. This current Q1i increases linearly with time.
At this time, since a voltage is generated also at the secondary winding 5b of the transformer T1, the switch Q4 is turned ON and hence the current Q4i flows clockwise through a path passing along 5b, Lo, Co, Q4, and 5b, so that electric power is supplied to the load RL. The current Q4i increases linearly with time, whereupon Lo(Io)2/2 of energy is stored in the smoothing reactor Lo. A current flowing into the smoothing rector Lo is depicted by Io.
When the switch Q1 is turned OFF, a reverse voltage is generated at the secondary winding 5b of the transformer T1, and hence the switch Q4 is turned OFF and the switch Q3 is turned ON. Therefore, the energy stored in the smoothing reactor Lo causes the current Q3i to flow clockwise through a path passing along Lo, Co, Q3, and Lo, so that electric power is supplied continuously to the load RL.
Subsequently, when the switch Q1 is turned ON, the voltage generated at the secondary winding 5b is reversed again, therefore the switch Q4 is turned ON and the switch Q3 is turned OFF, and then the similar operations as previously described are performed. Since the current of the smoothing reactor Lo flows continuously in the same direction, this state is called a continuous mode.
On the other hand, when a load current decreases (light load condition), a current of the smoothing reactor Lo flowing therethrough as the switch Q1 is turned OFF (e.g., time t32) becomes zero while the switch Q1 is OFF, but the switch Q3 remains ON. Therefore, the electric charge stored in the smoothing capacitor Co is discharged, and then a current Q3i′ flows counterclockwise through a path passing along Co, Lo, Q3, and Co, so that the energy is stored in the smoothing reactor Lo.
When the switch Q1 is turned ON at time t33 (same as time t31), the switch Q4 is turned ON and the switch Q3 is turned OFF. Therefore, a current Q4i′ flows counterclockwise through a path passing along Lo, 5b, Q4, Co, and Lo from the smoothing reactor Lo. As a result, the energy is finally returned to the DC power source vdc1 on a primary side (input side) via the primary winding 5a of the transformer T1.
As described above, when a synchronous rectifying circuit is applied to the conventional DC conversion apparatus shown in FIG. 1 or to a switching power source device described in Japanese Patent Application Laid-Open No. 2002-10636, they operate with little loss in a high load state in which a current flows continuously through the smoothing reactor Lo.